1. Field of the Invention
The present invention relates to a fan motor speed control circuit, a fan apparatus, and an electronic apparatus.
2. Description of the Related Art
An integrated circuit such as a CPU (Central Processing Unit) used for a server etc. increases in the amount of heat generated with increase in its operating speed. Since the CPU's increase in the amount of heat generated causes problems such as a thermal runaway, the server is generally provided with a fan for cooling the CPU. The rotation speed of the fan, namely, the rotation speed of a fan motor is generally controlled by a microcomputer etc., and an open control system (see, e.g., Japanese Patent Application Laid-Open Publication No. 2005-151792) and a feedback control system (see, e.g., Japanese Patent Application Laid-Open Publication No. 2007-68344) are known as a fan motor rotation speed control system.
The open control system (hereinafter, open control) is a system in which a driving circuit 500 drives a fan motor 510 based on a speed control signal for the fan motor 510 input from the microcomputer etc. as shown in FIG. 9. The speed control signal is for example a PWM (Pulse Width Modulation) signal and the rotation speed of the motor increases according to the duty ratio of an H level (high level) of the PWM signal.
The feedback control system (hereinafter, feedback control) is a system in which a fan motor 530 is driven so that the rotation speed of the fan motor 530 will match a target rotation speed, as shown in FIG. 10. To describe FIG. 10 in more detail, a reference voltage circuit 600 receives for example the PWM signal with the duty ratio corresponding to the target rotation speed of the fan motor 530 as a speed control signal from the microcomputer etc., and outputs a reference voltage Vref corresponding to the duty ratio of the H level. A speed voltage circuit 601 receives an FG (Frequency Generator) signal corresponding to the rotation speed of the fan motor 530 and outputs a speed voltage Vv corresponding to the FG signal. A comparison circuit 602 compares the reference voltage Vref and the speed voltage Vv and outputs a drive voltage Vdr as a comparison result. A motor driving circuit 603, based on the drive voltage Vdr, drives the motor 630 so that the speed voltage Vv will match the reference voltage Vref.
Recently, not only the prevention of the CPU thermal runaway etc. is performed, but also the reduction of power consumption of the fan motor and the fan motor speed control circuit and fan noise is performed by for example controlling the rotation speed of the fan motor according to an operation rate of the CPU. In the case of controlling the rotation speed of the fan motor according to the operation rate of the CPU as described above, it is preferable for the microcomputer controlling the rotation speed of the fan motor that the rotation speed of the fan motor is determined uniquely and linearly with respect to the duty ratio of the H level of the PWM signal, for example, as shown in a required characteristic of FIG. 11.
In the open control, as shown in FIG. 11, the rotation speed of the fan motor does not change linearly with respect to the duty ratio in the range between the duty ratio D1 and the duty ratio D2 due to for example the effect of the power consumption of the fan motor. Furthermore, in the case of the open control, the rotation speed of the fan motor varies due to variations of a power supply of the fan motor and manufacturing variations of the fan motor. Also, by a change of environment of an apparatus mounted with the fan, the fan motor, and objects to be cooled, system impedance indicating a pressure loss to an air flow within the apparatus mounted with the fan is caused to change, which results in a change of the rotation speed of the fan motor. Namely, in the open control, the rotation speed of the fan motor does not change uniquely and linearly with respect to the duty ratio of the H level of the PWM signal.
Accordingly, a preferable fan motor control system is the feedback control system that is insusceptible to effects of a change of the surrounding environment and by which the rotation speed of the fan motor changes uniquely and linearly with respect to the target rotation speed of the fan motor, namely, the duty ratio of the PWM signal to be input.
Incidentally, P-Q characteristic (air volume—static pressure characteristic) is among important characteristics of the fan and the fan motor. FIG. 12 is a diagram showing the P-Q characteristic, a relationship between the air volume and the rotation speed of the fan motor in the open control and the feedback control when the PWM signal with a predetermined duty ratio is input, where the air volume is taken as a horizontal axis, and the static pressure or the rotation speed of the fan motor is taken as a vertical axis. As shown in FIG. 12, under the open control, the rotation speed of the fan motor increases within a part of the range and this is caused by the effect of air resistance of the air sent by the fan.
On the other hand, within the above-mentioned part of the range under the feedback control: since the rotation speed of the fan motor is constant, the rotation speed of the fan motor is lower than that under the open control; and the static pressure is lower than that under the open control at the same air volume. The range within which the rotation speed of the fan motor under the open control is higher than that under the feedback control is defined as a range A.
As for a method of preventing lowering of the static pressure in the range A in the fan motor under the feedback control, there is a method of preventing deterioration of the P-Q characteristic by pre-programming the microcomputer so that the rotation speed of the fan motor will be higher in a deterioration range of the static pressure, as described in Japanese Patent Application Laid-Open Publication No. 2004-120980. However, the P-Q characteristic changes depending on the kind of fan, manufacturing variations of the fan and the fan motor, the duty ratio of the PWM signal as a speed control signal, etc. Therefore, the method of correcting the P-Q characteristic by pre-programming the microcomputer in the fan motor under the feedback control has a problem that the P-Q characteristic cannot be improved appropriately when changing from an expected characteristic.